_aa._ _PARMELIAE._ A somewhat complicated course of development has been traced by Baur[588] in _Parmelia acetabulum_. In that lichen the group of three to six carpogonia do not lie free in the gonidial tissue, but originate nearer the surface (Fig. 96) and are surrounded from the first by a tissue connected with, and resembling the tissue of the cortex. In the several ascogonia, there are more cells and more spirals than in _Collema_ or in _Physcia_, and all of them are somewhat confusedly intertwined. The trichogynes are composed of three to five cells and project 10 to 15µ above the surface. When further development begins, the ascogonial cells branch out and form a primary darker layer or hypothecium above which extends the subhymenium, a light-coloured band of loosely woven hyphae. Branches from the ascogonial hyphae at a later stage push their way up through this tissue and form above it a second plexus of hyphae—the base of the hymenium. Baur considers this a very advanced type of apothecium; he found it also present in _Parmelia saxatilis_, though, in that species, the further growth of the first ascogonial layer was more rapid and the secondary plexus and hymenium were formed earlier in the life of the apothecium. He has also stated that a similar development occurs in other genera such as _Usnea_, though Nienburg’s[589] work scarcely confirms that view. [Illustration: Fig. 96. _Parmelia acetabulum_ Dub. Vertical section of thallus and carpogonial group × 550 (after Baur).] In the brown _Parmeliae_, Rosendahl[590] found the same series of apothecial tissues, but he interprets the course of development somewhat differently: the basal dark layer or hypothecium he found to be of purely vegetative origin; above it extended the lighter-coloured subhymenium; the ascogenous hyphae were present only in the second layer of dark tissue immediately under the hymenium. In most lichens the primordium of the apothecium arises towards the lower side of the gonidial zone, the hyphae of which retain the meristematic character. In _Parmeliae_, as was noted by Lindau[591] in _P. tiliacea_, and by Baur[592] and Rosendahl[590] in other species, the carpogonial groups are formed above the gonidial zone, either immediately below the cortex as in _P. glabratula_, or in a swelling of the cortex itself as in _P. aspidota_, in which species the external enlargement is visible by the time the trichogynes reach the surface. In _P. glabra_, with a development entirely similar to that of _P. aspidota_, no trichogynes were seen at any stage. The position of the primordium close under the cortex is also a feature of _Ramalina fraxinea_ as described by G. Wolff[593]. The trichogynes in that species are fairly numerous.